1. Field of the Invention
The present invention relates to a photoelectric conversion element.
2. Description of the Prior Art
From the approximately past ten years ago, great attention has been paid to solar cells (photoelectric conversion element) employing silicon as a power source which is harmless to the environment. As for these solar cells employing silicon, a monocrystalline silicon type solar cell is known, which is used in artificial satellites or the like. In addition, as solar cells for practical applications, there are known a solar cell employing polycrystalline silicon (single crystal silicon) and a solar cell employing amorphous silicon. These solar cells have already been practically used in industrial and household applications.
However, since these solar cells employing silicon require high manufacturing cost and a great deal of energy in manufacturing thereof, thus these solar cells are not yet established as an energy-saving power source.
Further, since a dye-sensitized wet solar cell such as those disclosed in Japanese laid-open patent applications No. H01-220380, No. H05-504023 and No. H06-511113 employs an electrolyte of which vapor pressure is extremely high, there is a problem in that the electrolyte volatilizes.
For solving the problem, a perfect solid type dye-sensitized solar cell has been proposed (K. Tennakone, G. R. R. A. Kumara, I. R. M. Kottegoda, K. G. U. Wijiayantha, and V. P. S. Perera: J. Phys. D: Appl. Phys. 31(1998)1492). This solar cell is composed of an electrode on which a TiO2 layer is laminated and a p-type semiconductor layer provided on the TiO2 layer. However, this solar cell has a problem in that the p-type semiconductor layer is liable to penetrate the TiO2 layer to short-circuit the electrode.
Further, in the above proposal, CuI is used as a constituent material of the p-type semiconductor. The solar cell employing the CuI has a problem in that a generated current is lowered due to its deterioration caused by the increase in the crystal grain size of CuI and the like.
It is an object of the present invention to provide a solid type dye-sensitized photoelectric conversion element which is excellent in photoelectric conversion efficiency and which can be manufactured at a low cost.
In order to achieve the above object, the present invention is directed to a photoelectric conversion element, which comprises a first electrode; a second electrode arranged opposite to the first electrode; an electron transport layer arranged between the first electrode and the second electrode, at least a part of the electron transport layer being formed into porous; a dye layer which is in contact with the electron transport layer; a hole transport layer arranged between the electron transport layer and the second electrode; and short-circuit preventing means for preventing or suppressing short-circuit between the first electrode and the hole transport layer.
This makes it possible to provide a solid type dye-sensitized photoelectric conversion element having an excellent photoelectric conversion efficiency.
In the present invention, it is preferred that the short-circuit preventing means includes a barrier layer having a porosity smaller than the porosity of the electron transport layer. This makes it possible to more reliably prevent or suppress short-circuiting caused by electrical contact or the like between the first electrode and the hole transport layer, thereby enabling to effectively prevent the photoelectric conversion efficiency of the photoelectrical conversion element from being lowered.
In this case, it is preferred that when the porosity of the barrier layer is defined by A% and the porosity of the electron transport layer is defined by B%, the value of B/A is equal to or greater than 1.1. This makes it possible for the barrier layer and the electron transport layer to exhibit respective functions more appropriately.
More preferably, the porosity of the barrier layer is set to be equal to or less than 20%. This makes it possible to prevent or suppress the short-circuiting between the first electrode and the hole transport layer more reliably.
Further, it is preferred that the ratio of the thickness of the barrier layer with respect to the thickness of the electron transport layer is in the range of 1:99 to 60:40. This also makes it possible to prevent or suppress the short-circuiting between the first electrode and the hole transport layer more reliably. Further, it is also possible to effectively prevent the amount of light to be reached to the dye layer from being reduced.
Furthermore, it is also preferred that the average thickness of the barrier layer is in the range of 0.01 to 10 xcexcm. This also makes it possible to effectively prevent the amount of light to be reached to the dye layer from being reduced.
Moreover, it is also preferred that the barrier layer has electric conductivity which is substantially the same as that of the electron transport layer. This makes it possible to effectively move electrons from the electron transport layer to the barrier layer.
Moreover, it is also preferred that the barrier layer is mainly constituted from titanium oxide. This also makes it possible to effectively move electrons from the electron transport layer to the barrier layer.
Moreover, it is also preferred that the barrier layer is formed by means of a MOD method including a metal organic deposition and a metal organic decomposition. This makes it possible to easily and reliably obtain a barrier layer having a dense structure, that is having a desired porosity.
In this case, preferably the barrier layer is formed using a barrier layer material when the barrier layer is formed by means of the MOD method, in which the barrier layer material contains a metal alkoxide and an additive having a function for stabilizing the metal alkoxide.
Further, preferably, the additive is a hydrolysis suppressing agent that suppresses hydrolysis of the metal alkoxide by being replaced with alkoxyl group of the metal alkoxide and coordinated with the metallic atoms of the metal alkoxide.
Further, it is also preferred that the resistance value in the thickness direction of the total of the barrier layer and the electron transport layer is equal to or greater than 100 kxcexa9/cm2. This makes it possible to prevent or suppress the short-circuiting between the first electrode and the hole transport layer more reliably.
Furthermore, it is also preferred that the barrier layer is disposed between the barrier layer and the electron transport layer. This also makes it possible to prevent or suppress the short-circuiting between the first electrode and the hole transport layer even more reliably.
In this case, it is preferred that the boundary between the barrier layer and the electron transport layer is unclear. This makes it possible to reliably move electrons between the electron transport layer and the barrier layer.
It is also preferred that the barrier layer and the electron transport layer are integrally formed with each other. This also makes it possible to reliably move electrons between the electron transport layer and the barrier layer.
Further, it is also preferred that a part of the electron transport layer functions as the barrier layer. This also makes it possible to reliably move electrons between the electron transport layer and the barrier layer.
In the present invention, it is preferred that the short-circuit preventing means is a spacer which defines a space between the fist electrode and the hole transport layer. This makes it possible to more reliably prevent or suppress short-circuiting caused by electrical contact or the like between the first electrode and the hole transport layer, thereby enabling to effectively prevent the photoelectric conversion efficiency of the photoelectrical conversion element from being lowered.
In this case, it is preferred that when the average thickness of the spacer is defined by H xcexcm, the maximum thickness of the hole transport layer is defined by h1 xcexcm, and the total thickness of the electron transfer layer and the dye layer is defined by h2 xcexcm, they are configured so as to satisfy the relationship represented by the formula of h1+h2 greater than Hxe2x89xa7h1. This makes it possible to even more reliably prevent or suppress short-circuiting caused by electrical contact or the like between the first electrode and the hole transport layer.
In the present invention, it is also preferred that the dye layer functions as a light receiving layer which generates electrons and holes when receiving light. Such a dye layer is most preferable to the present invention.
In this case, it is preferred that the electron transport layer has an outer surface and a number of holes each having an inner surface, and the dye layer is formed on the outer surface of the electron transport layer as well as along the inner surfaces of the holes. This makes it possible to transport the electrons generated in the dye layer to the electron transport layer effectively.
Further, it is also preferred that the electron transport layer has at least a function that transports the electrons generated in the dye layer. Such an electron transport layer is most preferable to the present invention.
In the present invention, it is also preferred that the electron transport layer is formed into a film-like shape. This makes it possible to form the element into a thinner structure and to reduce its manufacturing cost.
Preferably, the average thickness of the electron transport layer is in the range of 0.1 to 300 xcexcm. This makes it possible to form the element into a thinner structure with maintaining the photoelectric conversion efficiency of the photoelectric conversion element appropriately.
In the present invention, it is also preferred that the porosity of the electron transport layer is in the range of 5 to 90%. This makes it possible to sufficiently enlarge the formation area of the dye layer. As a result, the dye layer enables to generate sufficient amount of electrons, and transport them to the electron transport layer with high efficiency.
In the present invention, it is also preferred that at least a part of the electron transport layer is formed of an electron transport layer material in the form of powder having the average particle size of 1 nm to 1 xcexcm. This makes it possible to obtain a porous electron transport layer more easily and reliably.
Further, it is also preferred that at least a part of the electron transport layer is formed of an electron transport layer material by means of a sol-gel method employing a sol liquid containing powder having the average particle size of 1 nm to 1 xcexcm. This also makes it possible to obtain a porous electron transport layer more easily and reliably.
In this case, it is preferred that the content of the powder of the electron transport material in the sol liquid is in the range of 0.1 to 10 wt %. This makes it possible to set the porosity of the electron transport layer appropriately.
In the present invention, it is also preferred that the electron transport layer is mainly formed of titanium dioxide. This enables the electron transport layer to enhance its electron transporting ability, and the electron transport layer itself becomes to generate electrons.
In the present invention, it is also preferred that the hole transport layer is mainly formed of a substance having ion conductive property. This makes it possible for the hole transport layer to transport holes generated in the dye layer effectively.
In this case, preferably, the substance having the ion conductive property is a metal halide compound. This also makes it possible for the hole transport layer to transport holes generated in the dye layer more effectively. More preferably, the metal halide compound includes a metal iodide compound. This also makes it possible for the hole transport layer to transport holes generated in the dye layer furthermore effectively.
Further, it is also preferred that the hole transport layer is formed by applying the hole transport material containing the substance having the ion conductive property onto the dye layer by means of a coating method. This enables to increase the contact area between the dye layer and the hole transport layer so that the hole transport layer can transport holes more effectively.
In this case, it is preferred that the hole transport layer is formed by applying the hole transport layer material onto the dye layer while the dye layer is being heated. This makes it possible to form the hole transport layer in a short time.
Further, it is also preferred that the hole transport layer material contains a crystal size coarse suppressing substance which suppresses increase in the crystal size of the substance having the ion conductive property when the substance crystallizes.
In this case, it is preferred that the content of the crystal size coarse suppressing substance in the hole transport layer material is in the range of 1xc3x9710xe2x88x926 to 10 wt %.
It is also preferred that the crystal size coarse suppressing substance includes thiocyanic acid salt, ammonium halide, or cyanoethylate.
Further, it is also preferred that the crystal size coarse suppressing substance suppresses the increase in the crystal size of the metal iodide compound when the metal iodide compound crystallizes by being bonded to the metallic atoms of the metal iodide compound.
Furthermore, it is also preferred that the hole transport layer material contains a hole transport efficiency enhancing substance that enhances the transport efficiency of the holes. This makes it possible for the hole transport layer to have enhanced hole transfer efficiency and improved electrical conductivity.
In this case, it is preferred that the content of the hole transport efficiency enhancing substance in the hole transport layer material is in the range of 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x921 wt %. This makes it possible to further enhance the hole transfer efficiency.
Further, it is also preferred that the hole transport efficiency enhancing substance includes a halide. More preferably, the halide is ammonium halide. This also makes it possible to further enhance the hole transfer efficiency.
In the present invention, it is preferred that the photoelectric conversion element further comprises a substrate for supporting the first electrode.
Further, in the present invention, it is also preferred that when the first electrode and the second electrode are applied with a positive and a negative voltage, respectively, with their difference being 0.5V, its resistance is larger than about 100 xcexa9/cm2. The fact that the photoelectric conversion element has such a characteristic as mentioned above means that occurrence of short-circuiting between the first electrode and the hole transport layer is effectively prevented or suppressed. Therefore, such a photoelectric conversion element can have more improved photoelectric conversion efficiency.
Furthermore, in the present invention, it is also preferred that when the photoelectric conversion efficiency for the angle of incidence of light on the dye layer of 90xc2x0 and 52xc2x0 (which are respectively an angle defined between the direction of the light and the surface of the semiconductor) are designated by R90 and R52, respectively, the ratio R52/R90 is larger than 0.8. The fact that the photoelectric conversion element can have such a characteristic as mentioned above means that the photoelectric conversion element has a low directivity for light, that is, it is isotropic to light. Accordingly, such a photoelectric conversion element can generate power more efficiently over almost entire range of shining period of the sun, if such a photoelectric conversion element is used outdoor.
Moreover, the photoelectric conversion element of the present invention is preferably applied to a solar cell. Although the photoelectric conversion element of the present invention may be applied to various devices or apparatuses, it is particularly preferably applied to a solar cell.
These and other objects, structures and advantages of the present invention will be apparent from the following detailed description of the invention and the examples taken in conjunction with the appended drawings.